It would be helpful to check your age theory with actual examples. I am not being snarky here. I'm genuinely curious about examples of starring pitchers turning around "age related" downward trends.
I've found that in general, the HR/9 increases over a pitchers career until age 35 from roughly .87 HR/9 to .96 HR/9, before dropping gradually over the next two years and then falling to .90 from ages 38-40. So a rising HR/9 over a player's career isn't necessarily a bad thing. Also, K/9 (and K%) drop every year until around age 36 before rising again. And BB/9 (and BB%) should fall consistently over the course of a players career. And, likewise, K/BB should usually rise throughout a player's career.
Ideally, you want a pitcher's decrease in walk rate to outpace his decrease in strikeout rate as he ages in order to see that K/BB to go up. And really, the key to pitching well and for a long time is to limit walks, which usually indicates good command of pitches. Naturally, a pitcher's stuff will decline as he gets older, but if he can learn to command (like Maddux for instance), then he will probably last longer in the league.
Edit: Also note that age related curves or studies have some bias. Younger pitchers are usually really good and put up good numbers, especially strikeouts. And older pitchers are usually pitchers who are around because they have great command and decent stuff. For instance, that HR/9 number drops probably because pitchers are still pitching in the league at age 38 already have good command, and can depress home runs by consistently hitting spots and pitching around hitters' strengths.
Edited by ScubaSteveAvery, 01 November 2012 - 02:04 PM.